A telecoil is a tightly wrapped piece of wire with sensitivity to magnetic flux. When a telecoil is installed in a hearing aid, the user can use the telecoil to pick up the voice signals from a speaker through inductive coupling. Many hearing aid users prefer to use the telecoil mode in a hearing aid because the voice coupling is inductive rather than acoustic. By using the telecoil mode, the user can reduce the background acoustic noise and feedback that can occur when using a phone with a hearing aid in microphone mode (i.e., when using the microphone of the hearing aid to pick up and amplify the sounds generated by the speaker). Because telecoils installed in hearing aids are physically small, the magnetic flux from a wireless handset speaker must be of sufficient strength to allow the hearing aid user to inductively couple to the voice signals.
Hearing aid users may experience audible interference when using a digital wireless telephone due to Electromagnetic Interference (EMI) from the phone. When EMI occurs in a hearing aid, it can produce an audible buzz that can make understanding speech difficult, communication over wireless phones annoying and—in the most severe cases—render the phone completely unusable to the hearing aid wearer. Fluorescent lights, electric toasters, electric motors and digital wireless telephones are examples of devices that can generate EMI, which then can be picked up by the telecoil in a hearing aid.
In the case of digital wireless phones, the EMI may be caused by several sources. One such source is the RF transmission envelope of a digital wireless phone. For example, a digital wireless transmission phone that employs a TDMA (Time Division Multiple Access) RF envelope, in which the RF Transmitter is turned on and off, generates a time varying RF field that appears as a pulsing field. This pulsing field can induce currents within the hearing aid, thereby producing an audible buzz. In addition, other components within the digital wireless telephone from the wireless phone's electronic elements such as its backlighting, display, keypad, battery and circuit board produce EMI, which can be picked up by the telecoil.
The Federal Communications Commission (FCC) requires that each digital wireless phone manufacturer provide wireless carriers with at least two commercially available wireless phone models that provide telecoil (e.g., magnetic) coupling capability for each wireless transmission technology. In addition, the FCC will require that wireless phones be tested for their telecoil coupling capability according to the ANSI C63.19 standard, which is incorporated herein by reference in its entirety. Wireless phone are considered passing (those with a rating of “good” or “excellent”) for use with hearing aids set in telecoil mode will be assigned a HAC (Hearing Aid Compatibility) rating of T3 or higher. The “T” is the designation for telecoil so that consumers will know the phone has been tested and rated for inductive coupling. The higher the “T” rating, the less likely the hearing aid user will experience interference when the hearing aid is set in the telecoil mode while using the wireless phone.